Interpretive Summary: The nutrional physiology of nickel (Ni), an essential trace element, is poorly understood in both plants and animals. This is especially true regarding its role as a cofactor in enzymes key to life essential metabolic processes. This research implicates a novel role for Ni in nitrogen (N) cycling within both plants and animals through its ability to convert a key N-cycling enzyme not requiring a metal cofactor (i.e., RNase A, a ribonuclease) for activity into a totally different enzyme (urease) exhibiting a different type of catalytic activity which converts urea to ammonium, with the ammonium serving as a key feedstock for N related metabolic needs of all living organisms. This provides greater fundamental understanding of the potential role and influence of Ni in N related metabolism of both plants and animals, potentially accounts for a nutritional and dietary role for Ni in life, and identifies a possible mechanism of action in regards to situations of Ni toxicity.

Technical Abstract:
Nickel (Ni) is an essential micronutrient; however, its metabolic or physiological functions in plants and animals are largely uncharacterized. The ribonucleases (RNase, e.g., RNase A) are a large family of hydrolases found in one form or many forms facilitating nitrogen (N) cycling. It is currently unknown how either a deficiency or excess of Ni influences the functionality of ribonucleases, like RNase A. This is especially true for perennial crops possessing relatively high Ni requirements. We report that the ‘rising’ xylem sap of pecan [Carya illinoinensis (Wangenh.) K. Koch, a long-lived tree] at bud break contains a 14 kDa RNase A (aka, RNase 1), which amount has a 33% greater in Ni-deficient as in Ni-sufficient trees when exposed to Ni ions exhibits ureolytic activity. The homologous 13.4 kDa bovine pancreatic RNase A likewise exhibits ureolytic activity upon exposure to Ni ions. Ni therefore affects enzymatic function of a typically non-metalloenzyme, such as it transforms to an enzyme capable of hydrolyzing a linear amide; thus, converting an endonuclease esterase into a urease. We conclude that Ni potentially affects the level and activity of RNase A present in the spring xylem sap of pecan trees, and probably in other crops, it has the same influence. The catalytic property of RNase A appears to shift from a nuclease to a urease relying on Ni exposure. This is suggestive that RNase A might possess novel metabolic functionality regarding N-metabolism in perennial plants. The ability of Ni to convert the activity of plant and animal RNase A from that of a ribonuclease to a urease indicates a possible unrecognized beneficial metabolic function of Ni in organisms, while also identifying a potential detrimental effect of excessive Ni on N related metabolic activity if there is sufficient disruption of Ni homeostasis.